Method for producing bipolar plates

ABSTRACT

Disclosed herein is a method for producing bipolar plates. The method comprises providing an electrically conductive sheet and then cutting through the sheet to create at least one opening for a fluid in the sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/816,609, filed on Jun. 16, 2010, which claims the benefit of thefiling date of U.S. Provisional Patent Application No. 61/334,379 filedMay 13, 2010, the disclosures of which are hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present generally concerns electrochemical fuel cells and moreparticularly to a method for fabricating bipolar plates.

Polymer electrolyte membrane or proton exchange membrane (PEM) fuelcells have intrinsic benefits and a wide range of applications due totheir relatively low operating temperatures (room temperature to around80° C., and up to -160° C. with high temperature membranes). The activeportion of a PEM is a membrane sandwiched between an anode and a cathodelayer. Fuel containing hydrogen is passed over the anode and oxygen(air) is passed over the cathode. The reactants, through the electrolyte(the membrane), react indirectly with each other generating anelectrical voltage between the cathode and anode. Typical electricalpotentials of PEM cells can range from 0.5 to 0.9 volts; the higher thevoltage the greater the electrochemical efficiency. However, at lowercell voltages, the current density is higher but there is eventually apeak value in power density for a given set of operating conditions. Theelectrochemical reaction also generates heat and water as byproductsthat must be extracted from the fuel cell, although the extracted heatcan be used in a cogeneration mode, and the product water can be usedfor humidification of the membrane, cell cooling or dispersed to theenvironment.

Multiple cells are combined by stacking, interconnecting individualcells in electrical series. The voltage generated by the cell stack iseffectively the sum of the individual cell voltages. There are designsthat use multiple cells in parallel or in a combination series parallelconnection. Separator plates (bipolar plates) are inserted between thecells to separate the anode reactant of one cell from the cathodereactant of the next cell. These separator plates are typically graphitebased or metallic (with or without coating). To provide hydrogen to theanode and oxygen to the cathode without mixing, a system of fluiddistribution and seals is required.

The dominant design at present in the fuel cell industry is to use fluidflow field plates with the flow fields machined, molded or otherwiseimpressed in the bipolar plates. An optimized bipolar plate has tofulfill a series of requirements: very good electrical and heatconductivity; gas tightness; corrosion resistance; low weight; and lowcost. The bipolar plate design ensures good fluid distribution as wellas the removal of product water and heat generated. Manifold design isalso critical to uniformly distribute fluids between each separator/flowfield plate.

There is an ongoing effort to innovate in order to increase the powerdensity (reduce weight and volume) of fuel cell stacks, and to reducematerial and assembly costs.

In a fuel cell system (stack & balance of plant), the stack is thedominant component of the fuel cell system weight and cost and thebipolar plates are the major component (both weight and volume) of thestack.

Bipolar plates are a significant factor in determining the gravimetricand volumetric power density of a fuel cell, typically accounting for 40to 70% of the weight of a stack and almost all of the volume. Forcomponent developers, the challenge is therefore to reduce the weight,size and cost of the bipolar plate while maintaining the desiredproperties for high-performance operation.

The multiple roles of the bipolar plate and the challenging environmentin which it operates means that the material from which it is made mustpossess a particular set of properties. The material should combine thefollowing characteristics:

High electrical conductivity, especially in the through-plane direction;

Low contact resistance with the gas diffusion layer (GDL)—depending onthe plate material and the thickness, the contact resistance with theGDL can be more important than the resistance of the plate itself;

Good thermal conductivity—efficient removal of heat from the electrodesis vital for maintaining an even temperature distribution;

Thermal Stability;

Gas impermeability to avoid potentially dangerous andperformance-degrading leaks;

Good mechanical strength—so as to be physically robust and to supportthe MEA;

Corrosion resistance—bipolar plates operate in a warm, damp environmentwhile simultaneously exposed to air and fuel over a range of electricalpotentials (ideal conditions for corrosion to occur);

Resistance to ion-leaching—if metal ions are released from the platethey can displace protons in the membrane and lower the ionicconductivity;

Thin and lightweight while accommodating the flow channels andmaintaining mechanical stability;

Low cost and ease of manufacturing;

Environmentally benign;

Recyclable.

A number of different methods have been used to manufacture bipolarplates including for example, U.S. Pat. No. 6,818,165 to Gallagher for“Method of Fabricating Fluid Flow Field Plates” on Nov. 16, 2004 andU.S. Pat. No. 6,997,696 to Davis et al for “Apparatus for CuttingExpanded Graphite Sheet Material” on Feb. 14, 2006. These methods,however, have a number of significant drawbacks. For example, thefabrication fluid flow field plates require four steps, namely rollerembossing fluid flow channels; reciprocally embossing fluid distributionareas; die cutting manifold openings; and curing the plates in anautoclave. The methods used to roller emboss flow channels, reciprocallyemboss fluid distribution manifolds, and then die cut the manifoldopenings requires careful alignment of the part between each of thesesteps. Additionally, four embossing dies and one cutting die per partare required, i.e., two mating roller dies, two mating reciprocal dies,and one cutting die to cut the manifolds. The methods use“pre-impregnated” expanded graphite which must be cured after partfabrication in an autoclave to improve the mechanical properties of thefluid flow field plate. The fluid distribution areas are not “straight”and therefore the roller is unable to emboss the entire part in onestep.

Thus, there is a need for an improved method for fabricating bipolarplates.

BRIEF SUMMARY OF THE INVENTION

We have designed a low cost method for producing lightweight bipolarplates. The method involves cutting through a sheet of flexible graphiteand then finishing the cut sheet. Unlike the examples described above,our method requires only die cutting flow channels and manifolds, andfinishing the part by pressing. Our method cuts all flow channels andmanifolds in one step and the “finishing” step does not require carefulpart alignment. Furthermore, our method only requires one cutting dieper part. The post cutting treatment is eliminated and since our methodof fluid distribution is effectively straight, we are able to cut allfeatures in one step with a rotary flexible die.

Accordingly, there is provided a method for producing bipolar plates,the method comprising providing an electrically conductive sheet; andcutting through the sheet to create therein at least one opening for afluid.

The method, as described above, further comprising finishing the cutsheet by pressing it between two rigid, flat plates.

In one example, the rigid, flat plates each include a non-stick coating.

The method, as described above, further comprising finishing the cutsheet by pressing it between two parallel rollers.

In one example, the parallel rollers each include a non-stick coating.

In another example, the cutting step is carried out using a die havingat least one blade. The die has two blades. The die is a rule die or aflexible die. The two blades of the die are located side-by-side.

In another example, the cut plate includes at least one elongate oxidantflow opening. The cut plate includes a plurality of elongate paralleloxidant flow openings. At least one oxidant inlet manifold opening andat least one oxidant outlet manifold opening located at the ends of theelongate oxidant flow openings and in communication therewith.

In another example, the cut plate includes at least one fuel inletmanifold opening and at least one fuel outlet manifold opening.

In another example, the cut plate includes at least one elongate fuelflow opening. The cut plate includes a plurality of elongate parallelfuel flow openings. The cut plate includes at least one fuel inletmanifold opening and at least one fuel outlet manifold opening which arelocated away and separate from the elongate parallel fuel flow openings.The cut plate includes at least one fuel inlet manifold opening and atleast one fuel outlet manifold opening located at the ends of theelongate fuel flow openings and in communication therewith. The openingscarry a coolant and fuel combination. The coolant and fuel combinationis an alcohol. The alcohol is methanol or ethanol. The alcohol ismethanol.

In one example, the cut plate is an oxidant flow field plate.

In another example, the cut plate is a fuel flow field plate.

In another example, the cut plate includes a plurality of oxidant inletmanifold openings and a plurality of oxidant outlet manifold openings.

In another example, the cut plate includes at least one fuel inletmanifold opening and at least one fuel outlet manifold opening.

In yet another example, the cut plate is a separator plate. Theseparator plate is a cooling fin separator plate.

In one example, the electrically conductive sheet is flexible graphite.

According to another aspect, there is provided a method of producing afuel cell stack, the method comprising: providing a Membrane ElectrodeAssembly (MEA) having an anode, a cathode and an electrolyte locatedtherebetween; locating an oxidant flow field plate against the cathode;locating a fuel flow field plate against the anode; locating a separatorplate against the oxidant flow field plate; compressing the plates toproduce the fuel cell stack, each of the plates each having been cutthrough to create a plurality of openings therein and finished, theplates being made from an electrically conductive material.

In one example, the step of compressing the plates creates a pluralityof oxidant flow channels between the oxidant flow field plate and thecathode, and a plurality of fuel flow channels between the fuel flowfield plate and the anode. The compressed plates are self-sealed.

In one example, the plates are made from flexible graphite.

In one example, in which prior to the compressing step, a gasket isbonded to each of the oxidant fuel field plate and the fuel flow fieldplate.

In another aspect, there is provided a stacked fuel cell assembly,comprising: a Membrane Electrode Assembly (MEA) having a cathode and ananode, and an electrolyte located therebetween; an oxidant flow fieldplate having a plurality of openings cut therethrough, the oxidant flowfield plate being located in intimate contact with the cathode so thatthe openings define a plurality of oxidant flow channels and a firstplurality of manifolds; a fuel flow field plate having a openings cuttherethrough, the fuel flow field plate being located in intimatecontact with the anode so that the openings define a plurality of fuelflow channels and a second plurality of manifolds; and a separator platelocated in intimate contact with the oxidant flow field plate, theseparator plate having a plurality of openings cut therethrough.

In one example, the channels are trapezoidal in cross section.

In one example, the plates are made from an electrically conductivematerial. The material is flexible graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of that described herein will become moreapparent from the following description in which reference is made tothe appended drawings wherein:

FIG. 1 is a cross-sectional view of unit cell assembly.

FIG. 2 is a top view of an oxidant flow field plate.

FIG. 3 is a top view of a fuel flow field plate.

FIG. 4 is a top view of a cooling fin separator plate.

FIG. 5 is a perspective top view of a bipolar plate assembly.

FIG. 6 is a perspective worm's eye view of a bipolar plate assembly.

FIG. 7 is a top view of a fuel flow field plate gasket.

FIG. 8 is a perspective top view of the fuel flow field plate and fuelflow field plate gasket assembly.

FIG. 9 is a perspective top view of the fuel cell stack assembly.

DETAILED DESCRIPTION

Definitions

Unless otherwise specified, the following definitions apply:

The singular forms “a”, “an” and “the” include corresponding pluralreferences unless the context clearly dictates otherwise.

As used herein, the term “comprising” is intended to mean that the listof elements following the word “comprising” are required or mandatorybut that other elements are optional and may or may not be present.

As used herein, the term “consisting of” is intended to mean includingand limited to whatever follows the phrase “consisting of”. Thus, thephrase “consisting of” indicates that the listed elements are requiredor mandatory and that no other elements may be present.

As used herein, the term “flow field plate” is intended to mean a platethat is made from a suitable electrically conductive material. Thematerial is typically substantially fluid impermeable, that is, it isimpermeable to the reactants and coolants typically found in fuel cellapplications, and to fluidly isolate the fuel, oxidant, and coolantsfrom each other. In the examples described below, an oxidant flow fieldplate is one that carries oxidant, whereas a fuel flow field plate isone that carries fuel. The flow field plates can be made of thefollowing materials graphitic carbon impregnated with a resin or subjectto pyrolytic impregnation; flexible graphite; metallic material such asstainless steel, aluminum, nickel alloy, or titanium alloy;carbon-carbon composites; carbon-polymer composites; or the like.Flexible graphite, also known as expanded graphite, is one example of asuitable material that is compressible and, for the purposes of thisdiscovery, easily cut through.

As used herein, the term “fluid” is intended to mean liquid or gas. Inparticular, the term fluid refers to the reactants and coolantstypically used in fuel cell applications.

Referring now to FIG. 1 in which a repeating unit cell assembly of afuel cell stack is shown generally at 10. The fuel cell 10 comprises aMembrane Electrode Assembly (MEA) 12, which includes an anode 14, acathode 16 and a solid electrolyte 18 located between the anode 14 andthe cathode 16. The MEA 12 is located between an oxidant flow fieldplate 20 and a fuel flow field plate 22. A first plurality of oxidantflow channels 24 are located between the oxidant field flow plate 20 andthe cathode 16 to supply the oxidant to the cathode 16. A secondplurality of fuel flow channels 26 are located between the fuel flowfield plate 22 and the anode 14 to supply fuel to the anode 14. Aplurality of oxidant channel landings 28 are located on one side of theoxidant flow field plate 20, and fuel channel landings 30 are located onone side of the fuel flow field plate 22 and respectively intimatelycontact the cathode 16 and the anode 14 to allow the passage ofelectrical current and heat from the MEA 12. A separator plate 32 islocated in intimate contact with the oxidant fuel field plate 20 andallows the axial passage of electrical current therealong. In theexample illustrated, the separator plate 32 is a cooling fin separatorplate which also laterally transfers heat to external cooling fins andacts as a separator between each repeating unit cell. Typically, whenmultiple cell are assembled, each fuel flow field plate 22 lies inintimate contact with a cooling fin separator plate 32, thereby sealingthe channels 26.

Referring now to FIG. 2, an individual oxidant flow field plate 20 isshown and includes at least one elongate oxidant flow channel 34. In theexample shown, a plurality of elongate parallel oxidant flow openings 34are cut through the plate 20 and extend parallel to each other along thecentral portion of the plate 20. Each elongate oxidant flow openingincludes an oxidant inlet manifold opening 36 and an oxidant outletmanifold opening 38, which are located at each end of the elongateoxidant flow opening 34. The oxidant flow field plate 20 also includes aperipheral area 40, which forms a boundary around the elongate oxidantflow openings 34. A fuel inlet manifold opening 42 and a fuel outletmanifold opening 44 are cut through the peripheral area 40 and arelocated away from each other on opposite sides of the oxidant flowopenings 34. A plurality of holes 46 to accommodate a stack compressionsystem are also cut through the peripheral area 40, as will be explainedbelow

Referring now to FIG. 3, an individual fuel flow field plate 22 is shownand includes at least one elongate fuel flow opening 48. In the exampleillustrated, a plurality of elongate parallel fuel flow openings 48 arecut through the plate 22 and extend parallel to each other along thecentral portion of the plate 22. The plate 22 includes a peripheral area50, which forms a boundary around the elongate fuel flow openings 48. Aplurality of oxidant inlet manifold openings 52 are located away fromone end of the elongate fuel flow openings 48 and a plurality of oxidantoutlet manifold openings 54 are located away from the other end of theelongate fuel flow openings 48. Each of openings 52 and 54 isindividually cut through the plate 22 and form discrete openings and arenot connected to the elongate fuel flow openings 48. A fuel inletmanifold opening 56 and a fuel outlet manifold opening 58 are cutthrough the peripheral area 50 and are located away from each other onopposite sides of the fuel flow openings 48. Fuel inlet manifold opening56 is connected to fuel flow openings 48 via a slit in an adjacentpolyethylene terephthalate (PET) gasket system which partially coversperipheral area 50, which will be described below. Similarly, fueloutlet manifold opening 58 is connected to fuel flow openings 48 in thesame manner. A plurality of holes 60 to accommodate a stack compressionsystem is also cut through the peripheral area 50, as will be explainedbelow. In one example, the openings 48 can carry a coolant and fuelcombination, which is an alcohol. In one example, the alcohol is eitherethanol or methanol, although methanol is typically used.

Referring now to FIG. 4, an individual cooling fin separator plate 32 isshown and includes a plurality of oxidant inlet manifold openings 62 anda plurality of oxidant outlet manifold openings 64 which are locatedaway from the openings 62 at opposite ends of the plate 32 and are cutthrough the plate 32. A fuel inlet manifold opening 66 and a fuel outletmanifold opening 68 are cut through the plate 32 and are located awayfrom each other. A plurality of holes 70 to accommodate a stackcompression system is also cut through the plate 32, as will beexplained below. Two cooling fin areas 72, 74 are located on either sideof the openings 62, 64, 66, and 68 and holes 70.

Referring now to FIGS. 1, 5 and 6, when the plates 20, 22 and 32 areassembled, the elongate oxidant and fuel flow field openings 34, 48 arealigned to create the oxidant flow channels 24 and the fuel flowchannels 26. The cooling fin areas 72, 74 extend away from the plates20, 22 along substantially the entire length of the plates 20, 22. Inthe example shown, the manifold openings 36, 38, 42, and 44 align withthe corresponding manifold openings in the cooling fin separator plate32. One of the advantages of using flexible graphite plates tomanufacture the fuel cell stacks is that they are self-sealing whencompressed together to form the stack. This eliminates the need forbonding using adhesives during the assembly of the plates.

Referring now to FIGS. 3, 7 and 8, when plate 22 is assembled to a PETgasket system 80, the manifold openings 52, 54, 56 and 58 align withmanifold openings 82, 84, 88 and 92, respectively. Fuel from fuel inletmanifold 56 travels though fuel inlet manifold 88, and down slitmanifold 90 made in the PET gasket. Fuel then passes down fuel flowfield channels 48, reacting with active area 88, to slit manifold 94 andthen exiting from fuel outlet manifold 92. A similar gasket system isalso applied to cathode flow field plate 20, although the slit manifolds90 and 94 are omitted.

Referring now to FIGS. 1 and 9, where unit cells 10 are stacked betweenanode end plate 102 and cathode end plate 100, compressed, and fastenedwith compression system 104, which typically includes fasteners such asa nuts and threaded studs.

The fuel cell stacks described herein are particularly well suited foruse in fuel cell systems for unmanned aerial vehicle (UAV) applications,which require very lightweight fuel cell systems with high energydensity. Other uses for the fuel cell stacks include auxiliary powerunits (APUs) and small mobile applications such as scooters, which alsorequire lightweight systems. Indeed, the fuel cell stacks may be usefulin many other fuel cell applications such as automotive, stationary andportable power.

Manufacturing Process—Prototype Level

Flexible graphite is used to produce the fuel flow field plate 22, theoxidant flow field plate 20 and the cooling fin separator plate 32 andcan be purchased in roll form.

Flexible dies or rule dies used in the cutting process, available frommany die manufacturers, are typically used for label cuttingapplications and generally can cut hundreds of thousands of plates. Theflexible die design is dependent on feature geometry and materialthickness. The cutting step is typically carried out using a die havingat least one blade, which cuts through the flexible graphite sheet andpushes the scrap away from the opening. In one example, the die has twoblades located side-by-side and which cut through the sheet such thatthe scrap is removed from the sheet as a single piece.

Typically, for the fuel flow field plate 22, a 0.015″ thick sheet isused.

Typically, for the cooling fin separator plate 32, a 0.015″ thick sheetis used.

Typically, for the oxidant flow field plate 20, a 0.020″ thick sheet isused.

Cutting

The fuel flow field plate 22, the oxidant flow field plate 20 and thecooling fin separator plate 32 are individually cut through using theirrespective flat, flexible dies using a manual, reciprocal hydraulicpress.

The press cutting force varies from 6000 lbs to 11,000 lbs, which ismonitored with a pressure gauge, and which depends on the number andspacing of die features. Thus, a tightly packed die with many featuresrequires a greater cutting force.

Once cut through, the plates 20, 22 and 32 are removed from the die withsuboptimal feature definition, part deformation and jagged edges wherethe die cutter penetrated the flexible graphite material. The scrapmaterial that is removed during the cutting can be recycled. The diesare selected such that they cut the specific flow openings and manifoldopenings in the plates, as illustrated in FIGS. 2, 3 and 4.

Finishing

After cutting through, each plate is then pressed between two flat,rigid, parallel plates in the same manual hydraulic press to improvefeature tolerance, eliminate undesired deformation caused by the die,and to “flatten” rough, jagged edges left by the cutting process.

A thin layer of Teflon is the applied to the pressing fixture on eitherside of the plates to improve surface finish and to eliminate “sticking”The cut through plates 20, 22 and 32 are then ready for stack assembly.

Manufacturing Process—Production Level

For higher volume manufacturing, rotary die cutting is used forincreased throughput. Rotary flexible dies are available from many diemanufacturers. Cylindrical flexible dies are mounted on a magneticcylinder and mate with a cylindrical anvil, where all three dies can usethe same magnetic cylinder to reduce cost. Rotary die cutting equipmentfor the label making industry is used.

Flexible graphite material (available in rolls) is automatically fedinto the equipment. Typically, 3000 plates per hour are potentiallypossible using this manufacturing method.

Cutting

The fuel flow field plate 22, the oxidant flow field plate 20 and thecooling fin separator plate 32 are individually cut through using theirrespective rotary, flexible dies using rotary die cutting equipment. Thedistance between the rotary die and anvil is adjusted to achieve optimalpart cutting. An automated scrap removal system removes residualflexible graphite for recycling.

A plate handling system, typically a conveyor, groups and transports thecut through plates to the “finishing” area.

Finishing

Each cut through plate is automatically fed into a rotary flatteningsystem which comprises of two parallel rollers with Teflon coating andadjustable spacing. The finished plates are automatically removed fromthe rollers via conveyor and transported to their respective part bins.The plates are then ready for stack assembly.

Stack Assembly

After the three plates 20, 22, and 32 of the bipolar plate are“finished”, a perimeter gasket made of PET is bonded to the oxidant flowfield plate 20 and fuel flow field plate 22. The gasket mates to eitherside of the MEA 12 and provides a gas tight seal. As described above,the PET gasket bonded to the anode flow field plate incorporates a slitmanifold 90 which routes fuel from the fuel inlet manifold 56, to fuelflow openings 48, and from fuel flow openings 48, along slit manifold94, and out fuel outlet manifold 92. The stack is assembled one plate ata time beginning with anode end plate 102. The anode flow field plate 22is placed first (with the PET gasket attached), the MEA 12 is thenadded, and then the oxidant flow field plate 20 (with the gasketattached). The next step is to add the cooling fin separator plate 32.This set of steps is repeated a plurality of times to build a stack. Thelast step is to add cathode end plate 100. This assembly is thencompressed in a fixture and a compression system 104 holds thecompressed stack together. Threaded rod studs pass through the holes forstack compression 46, 60, and 70, which are then fastened with a nut atthe other end.

Since material is cut away from the bipolar plate, the fluid flowchannels and manifolds are designed in such a way so that they aresupported around their perimeter. Straight, parallel fluid flow channelsand round manifolds are especially suited for this approach; serpentineflow channels which are sometimes used in fuel cells are not assuitable, although supporting tabs can be added where the tabs areembossed in a second manufacturing step.

For each plate, all features are cut simultaneously (channels andmanifolds). It is possible to sequentially cut the plate features, butan additional die and alignment jig is required, which adds fabricationtime and increases cost.

Alternatives

A unitary body would be fabricated using the method as described above,but would then be mechanically or adhesively bonded together by pressingforce, or using silicone adhesive, respectively; this would create abipolar plate.

A “hydrid” laminate structure is also contemplated which may includeflexible graphite fluid flow channels, and an aluminum cooling fin.These subcomponents could also be mechanically or adhesively bondedtogether to create one part. In this case, the adhesive would not beapplied to the active area portion of the bipolar plate because it isnot electrically conductive.

An alternative approach would be would be to assemble the entire unitcell, as illustrated in FIG. 1, using adhesive as opposed to assemblingthe bipolar plate. In this case, the MEA 12 would be adhesively bondedto the plates 20 and 22, and then the cooling fin separator plate 32would be bonded to the plate 20. This would allow testing on a unit celllevel to confirm good performance and sealing characteristics prior tostack integration. The thickness of each plate and adhesive in the unitcell would need to be controlled so that good conductivity and sealingwould be maintained.

Alternatively, a liquid cooling capacity is also contemplated in whichthe cooling fin separator plate is omitted and a liquid cooling sectionintegrated in the stack. This design would require relocation of themanifolds to account for the additional fluid.

The “finishing” stage of the part fabrication could be used to increasethe density of the flexible graphite and therefore improve mechanicaland electrical properties (i.e. a 0.020″ thick cut part could be presseddown to 0.015″).

The anode flow field plate uses a “hybrid” manifold design, whereas thefluid link between the fuel manifold and anode flow field is achievedvia an embossed feature in the cooling fin separator plate instead ofthe slit manifold in the PET gasket currently used, but this would add afabrication step for this plate.

The plates can be fabricated with high volume manufacturing process(reciprocal or rotary die-cutting commonly used in label making)therefore reducing overall part cost;

Parts can be fabricated using very low cost tooling (flat or cylindricalflexible dies). Moreover, flexible graphite raw material is inexpensiveand is available in various forms and thicknesses. Advantageously, sealsare eliminated between layered bipolar plate components due to sealingnature of flexible graphite, which reduces part count and thereforeoverall cost. Flexible graphite has a typical density of 1.12 g/cc. Puregraphite typically used for machining bipolar plates has a density ofapproximately 2.0 g/cc (1.79 times more). Graphite used for moldedbipolar plates can achieve a density as low as 1.35 g/cc (1.2 timesmore) but requires expensive injection molding equipment and cavitydies. Additionally, flexible graphite bipolar plates fabricated viadie-cutting have reduced mass because material is removed for flowchannels and manifolds as opposed to being embossed.

Fluid flow channel depth may be changed easily by changing thickness offlexible graphite sheet and using same die. Also, a modular bipolarplate allows for various fuel cell configurations, for example, if morecooling is required for a specific application, a larger air cooling fincan be substituted.

Modular bipolar plate also provides ability to preassemble unit cell foreasier stack assembly. Testing can also be performed on this unit cellprior to stack assembly.

Modular bipolar plate also allows supplemental cooling channels forliquid cooling to be integrated between unit cell assemblies for otherstack designs (i.e. fluid manifolds and flow channels would requireredesign).

Resulting bipolar plate is very thin (i.e. 0.015″+0.015″+0.020″=0.050″thick) which reduces overall volume.

Cathode and anode fluid flow channels are perpendicular, reducing thecomplexity of fuel manifolding.

Cathode and anode fluid flow channels are perpendicular and integratedinto a square stack design, thereby allowing the cathode flow fieldplate and anode flow field plate to be identical and cut with the sameflexible die.

Cathode and anode fluid flow channels are perpendicular and integratedinto a square stack design, thereby allowing the cathode flow fieldplate and anode flow field plate to be identical and cut with the sameflexible die, where the channel opening size is adjusted via thematerial thickness.

Cathode and anode fluid flow channels are perpendicular and integratedinto a square stack design, where the cooling fin separator plate doesnot incorporate cooling fins, where the cooling capacity is performedvia a liquid fuel in the anode flow channels such as methanol for aDirect Methanol Fuel Cell (DMFC).

Other Embodiments

From the foregoing description, it will be apparent to one of ordinaryskill in the art that variations and modifications may be made to theembodiments described herein to adapt it to various usages andconditions.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for producing bipolar plates, the method comprising: (a)providing an electrically conductive sheet; and (b) cutting through thesheet to create therein at least one opening for a fluid.
 2. The methodaccording to claim 1, further comprising finishing the cut sheet bypressing it between two rigid, flat plates.
 3. The method according toclaim 2, in which the rigid, flat plates each include a non-stickcoating.
 4. The method according to claim 1, further comprisingfinishing the cut sheet by pressing it between two parallel rollers. 5.The method according to claim 4, in which the parallel rollers eachinclude a non-stick coating.
 6. The method according to claim 1, inwhich the cutting step is carried out using a die having at least oneblade.
 7. The method according to claim 6, in which the die has twoblades.
 8. The method according to claim 7, in which the two blades ofthe die are located side-by-side.
 9. The method according to claim 6, inwhich the die is a rule die or a flexible die.
 10. The method accordingto claim 1, in which the cut plate includes at least one elongateoxidant flow opening.
 11. The method according to claim 10, in which thecut plate includes a plurality of elongate parallel oxidant flowopenings.
 12. The method according to claim 11, in which at least oneoxidant inlet manifold opening and at least one oxidant outlet manifoldopening located at the ends of the elongate oxidant flow openings and incommunication therewith.
 13. The method according to claim 1, in whichthe cut plate includes at least one fuel inlet manifold opening and atleast one fuel outlet manifold opening.
 14. The method according toclaim 1, in which the cut plate includes at least one elongate fuel flowopening.
 15. The method according to clam 14, in which the cut plateincludes a plurality of elongate parallel fuel flow openings.
 16. Themethod according to claim 15, in which the cut plate includes at leastone fuel inlet manifold opening and at least one fuel outlet manifoldopening which are located away and separate from the elongate parallelfuel flow openings.
 17. The method according to claim 15, in which thecut plate includes at least one fuel inlet manifold opening and at leastone fuel outlet manifold opening located at the ends of the elongatefuel flow openings and in communication therewith.
 18. The methodaccording to claim 15, in which the openings carry a coolant and fuelcombination.
 19. The method according to claim 18, in which the coolantand fuel combination is an alcohol.
 20. The method according to claim19, in which the alcohol is methanol or ethanol.
 21. The methodaccording to claim 20, in which the alcohol is methanol.
 22. The methodaccording to claim 1, in which the cut plate is an oxidant flow fieldplate.
 23. The method according to claim 1, in which the cut plate is afuel flow field plate.
 24. The method according to claim 1, in which thecut plate includes a plurality of oxidant inlet manifold openings and aplurality of oxidant outlet manifold openings.
 25. The method accordingto claim 1, in which the cut plate includes at least one fuel inletmanifold opening and at least one fuel outlet manifold opening.
 26. Themethod according to claim 1, in which the cut plate is a separatorplate.
 27. The method according to claim 26, in which the separatorplate is a cooling fin separator plate
 28. The method according to claim1, in which the electrically conductive sheet is flexible graphite.